Field Replaceable Units
Many electronic systems, including most computer systems, contain multiple field replaceable units (FRUs). FRUs generally include any portion of an electronic system that is designed to be replaced without requiring transport of the entire system to a repair facility. FRUs include input/output cards and processor modules, including PCI bus cards, of computer systems. FRUs also include channel interface cards of telephone switching and other communications systems.
As with anything else built by man, electronic circuitry can fail. Electronic systems, including computers, are often repaired by replacing one or more FRUs. FRUs may also be added to a system, or exchanged with others in a system, to reconfigure or expand the system to meet particular system requirements.
Hot Plugging
It is often undesirable to completely shut down an electronic system for maintenance, even when maintenance requires replacement of, or addition of, one or more FRUs. For example, it is undesirable to shut down a telephone switching machine serving ten thousand customers so that a trunk interface card can be replaced. Similarly, it is undesirable to shut down an entire airline reservation-tracking computer system for minor repairs and reconfiguration. Many electronic communications and computing systems therefore allow hot-plugging (also known as hot-socketing) of FRUs to minimize the need for system shutdowns during repair and reconfiguration.
An example hot-pluggable FRU is a PCMCIA expansion card such as are commonly used with notebook computers. PCMCIA cards have a connector supporting moderately high-speed digital interconnect in the form of a parallel digital bus, as well as power, control, and reset connections.
High Speed Interconnect
Many FRUs of modem communications and computing systems have connectors supporting one or more high-speed digital interconnect systems. These high speed interconnect systems typically involve one or more parallel busses, such as the PCI or PCMCIA busses, allowing for two, three or more connections. Many other bus types are also known. High speed interconnect may also be point-to-point interconnect having two connections.
FRUs may incorporate processors and/or memory. They may also incorporate input-output (IO) devices such as network interfaces, disk drives, disk drive controllers, display and keyboard adapters, power supplies, and many other components of communications and computing systems.
Designs are known for systems wherein at least some FRUs can be exchanged while other components of the system continue operation. For example, many RAID (Redundant Array of Independent Disks) array systems provide for replacement of failed drives and reconstruction of datasets without requiring system shutdown. These systems often provide mechanisms for sequencing power and reset connections to an FRU. These designs also often provide mechanisms for self testing each FRU after it is inserted into a system.
JTAG
The IEEE 1149.1 serial bus, also known as the JTAG bus, was devised for testing of inactive FRUs by providing access from a tester to circuitry within the FRU. In particular, the JTAG bus provided ability to perform a boundary scan on each integrated circuit on an FRU. The tester can verify connectivity of the integrated circuits of an FRU and verify that they are installed correctly. The JTAG bus provides for interconnection of one or more integrated circuits in a chain, any of which may be addressed by the tester. Typically, multiple devices of a circuit board are interconnected into a JTAG chain.
The JTAG bus uses four wires. These include a serial data-in line, a serial data-out line, a clock line, and a test mode select line. Typically the data-out line of a first chip in a chain couples in daisy-chain configuration to the data-in line of a second chip of the chain, and the data-out line of the second chip couples to the data-in line of a third; the data-out line of the last chip in the chain is brought back to the test connector.
The IEEE 1152 bus is a newer, enhanced, version of the 1149.1 JTAG bus. References herein to a JTAG bus are intended to include both the 1149.1 and 1152 variations.
The JTAG bus is most often used for testing an FRU in a factory environment, typically when these FRU's are inserted into FRU test apparatus for production testing. For purposes of this application, the term system excludes FRU test apparatus as used in production testing; the term system includes computer systems where FRUs operate to run operating system and user programs.
Installation of FRUs
When FRUs are inserted into a system, it is possible that some wires of connectors may make proper contact with circuitry of the FRU while other wires may not couple correctly—they may be resistive or remain open. This is particularly likely if the connectors are dirty, or if circuit boards of the system and FRU flex during insertion. If the connections coupling the FRU to other parts of the system can be tested for resistive and open wires, an installer could repair the installation by cleaning the connectors and reseating the FRU.
Newly installed FRUs may also have cold solder joints or electrostatic discharge (ESD) damage that can also impair communications over connections coupling the FRU to other parts of the system. While cold solder joints and ESD damage can not be repaired by cleaning connectors, it is desirable to identify FRUs having these faults and avoid using them in systems.
In modern high performance systems, error correcting coding (ECC) may be used on some high speed interconnect, including high speed interconnect crossing connections between an FRU and remaining parts of the system. ECC can, however, mask the effect of resistive or open wires of connectors coupling an FRU to remaining parts of the system. This masking occurs because the ECC makes the system appear to work correctly even with resistive or open wires. It is desirable to identify resistive and open wires of connectors protected by ECC since resistive and open wires can cause other faults, normally correctable through ECC, to be uncorrectable; thereby degrading system reliability
It is therefore desirable to test connections between an FRU and remaining parts of a system upon installation or replacement of an FRU.